It is an email petition plugin, a simple form. I could not get the submit button to work, it remained unclickable, because the related code from the plugin files was being blocked somehow, don't ask me how. In the end it was solved by pasting part of the plugin code to suffusion's sidebar.php.

Now, I am wondering if this is a known issue, if the suffusion sidebar blocks code by plugins.

It may be a code conflict between the theme and your plugin. There is no way that the theme can be tested with all plugins likewise there is no way a plugin can be tested with all themes. These conflicts do occur from time to time.